This invention relates to telephone holding circuits, and more particularly to an automatic release telephone hold circuit for a series of telephones connected to a telephone line.
Telephones with hold features are well known. Generally, these devices are provided by the telephone company for use on multiline telephones having a plurality of push buttons on the phone. The hold circuitry involved in such devices is generally complex in construction and requires special costly installation. The telephone company charges an initial installation fee for providing the hold equipment and also charges a monthly fee for such service. The known types of hold devices are of the mechanical push button type used for both applying the hold condition and for releasing the hold condition. Specifically, to establish a hold condition, the hold button is pressed down and immediately the telephone is put on hold while making the telephone handset inoperative for voice communication. To release the hold condition and restore the voice communication, a second mechanical button must be depressed.
Because of this excessive cost involved in providing a telephone with a hold feature and because of the complexity of the standard telephone hold equipment, numerous other types of hold circuits have been provided and are readily available. The other types of hold circuits respond to changes in voltage level on the telephone line to thereby detect an on-hook and off-hook condition of an associated telephone instrument. The hold circuit, upon actuation, will either maintain or release its hold mode in response to its detection of such telephone line voltage level changes. However, this detection capability differentiating between on-hook and off-hook voltage levels by prior art hold circuits, requires that the phone instrument be on-hook in order that the hold circuit maintain its hold mode subsequent to its actuation. More specifically, to activate the hold circuit an actuating control switch after being initially depressed, must be continuously retained in its depressed condition until after the telephone handset is replaced on-hook. And, only after that time, will the higher on-hook voltage cause a latching circuit to maintain the hold mode on the phone line and only then may the control switch be released. These types of prior art hold circuits therefore, in addition to presenting an inconvenience because of the aforementioned requirement to maintain the hold button depress D, are also prone to inadvertent disconnection of the far-end party if the hold button be released prematurely.
Another difficulty with many prior art hold circuits is that they are not automatically released when a telephone is picked up. With many such hold circuits, it is necessary to go back to the same telephone on which the hold was set in order to release the hold condition. Therefore, in a situation were a user wants to place a telephone on hold so that he can go to another location and pick up an extension, after completing the telephone call he must return back to the original telephone in order to release the hold condition. This inconvenience has also prevented widespread acceptance of these thpes of hold circuits.
Yet another type of available hold circuit responds to the changes in voltage levels and also provides an intermediate voltage level in between the on-hook and off-hook voltages, in order to latch the hold condition. This type of circuit in addition to requiring that the actuation control switch be maintained depressed as above mentioned, has extremely limited use because the actual voltages on a telephone line will fluctuate over a wide range. The typical -50 volts applied at a central office can vary to as low as -25 volts, or even less, when the telephone is located at a substantial distance from the central office. At the same time, the off-hook voltage can vary from as little as -3 volts to as much as -10 volts, and in some cases -15 volts. Accordingly, the ranges tend to approach each other providing little intermediate values. As a result, circuits responsive to an intermediate voltage level can produce erroneous results because of the variations and fluctuations of the telephone line voltages existing throughout various locations.
A further disadvantage of the above mentioned hold circuit is its requirement for continuous line current drain in its standby mode.
An improved hold circuit which avoids most of the aforementioned problems is described in my U.S. Pat. No. 4,001,520, issued Jan. 4, 1977. In that patent there is described an improved hold circuit which includes a relay having a coil and normally open contacts. A normally opened momentary contact switch is connected in series with the relay coil, and the series combination is connected across the telephone lines. The contact switch is connected in parallel to the normally opened contacts of the relay. The hold circuit captures or seizes the telephone lines when the contact switch is closed, and permits the telephone lines to be released upon removing the telephone handset offhook. A parallel combination of a lamp and a capacitor is connected in series between the relay contacts and the relay coil to ensure automatic release of the telephone line when any of the telephones connected to the lines are placed offhook. An alternate circuit which substitutes a semiconductor element for the relay is also shown in my above mentioned patent.
The hold circuit described in my aforementioned patent provides unique benefits not achieved by any of the prior hold circuits. For example, in my described improved hold circuit it is not necessary to maintain the button while the telephone is replaced on-hook. On the contrary, momentary depression of the hold button immediately establishes a hold condition even if the telephone is retained off-hook. Furthermore, even though the hold condition has been established, the telephone handset still remains active and is still in voice communication with the telephone line until it is actually placed back on-hook. However, the hold circuit retains the line seized.
Additional benefits of the aforementioned hold circuit include the automatic release of the hold condition upon pick-up of any of the extension phones from its respective hook. Also, when the far-end party to the conversation disconnects the telephone line, the disconnect pulse removes the hold condition. Furthermore, because of the use of the lamp in the load circuit, there is provided a visual indication of the operational status of the hold circuit prior to replacing the telephone on-hook. The lamp can also provide a visual indication when the circuit releases the hold condition, and can also provide an indication of eavesdropping on the line, thereby ensuring the privacy of the conversation.
Further benefits are provided by utilizing the aforementioned hold circuit, in that in standby, its line current drain is zero. Also, a great range of voltage fluctuations can be accommodated by the circuit, and it still operates effectively. All of the aforementioned prior art systems require that each telephone instrument have some form of actuating control switch as an add-on component to the phone instrument in order that same be conveniently accessible to the user. Moreover, in all of the aforementioned prior art systems, multiple telephone instruments, although connected to a common phone line would necessitate providing a separate and complete hold device for each individual instrument of the multiple telephone group. The above constitutes two distinct shortcomings and major disadvantages of the prior art systems.
The subject invention overcomes both of the above disadvantages. In the instant invention, the telephone instrument call-out means, i.e., the rotary dial or push button pad, is employed as the actuating means to activate the hold circuitry.
Although my aforementioned hold circuit provides unique and improved benefits over prior art systems, as described in the aforementioned patent, nevertheless, it is necessary to have a separate hold circuit for each telephone extension connected on a particular telephone line. Thus, in a typical system having a single telephone line with multiple telephone units connected to that line, each of the telephone units would require its own hold circuit of the type described. It would therefore be beneficial to avoid the necessity of multiple holding circuits when a large number of telephones are connected to the same telephone line.
U.S. Pat. No. 4,011,413 issued Mar. 8, 1977, attempts to solve this problem by providing a single sensing and latching circuit connected across the telephone line with a separate actuating control switch connected to each telephone unit within the system. The single sensing and latching unit senses the on-hook and off-hook voltage levels, and also responds to an intermediate voltage level between the high on-hook voltage and the low off-hook voltage. Each of the individual control switches affects the intermediate voltage level for detection by the single sensing and latching unit. However, in order to achieve the intermediate voltage level, it is necessary to depress the control switch and maintain it in a depressed condition while the telephone handset is replaced into its on-hook condition. It is impossible to only momentarily actuate the control switch in order to establish the hold condition. The depressed control button must be so maintained until the receiver is back on-hook before the intermediate voltage level is generated to cause the latching unit to seize the line into a hold condition. Moreover, each extension station must have its own individual add-on custom design control switch associated with it in order to be able to activate the hold circuit. Furthermore, the intermediate voltage level is only applicable for limited telephone systems where the intermediate range between the on-hook voltage and the off-hook voltage is sufficiently wide. However, because of the wide fluctuations of telephone voltages, in most practical situations it will be extremely difficult to accurately sense the intermediate voltage level making it extremely difficult to achieve universal accurate and consistently reproducible hold conditions.